Rock drill



J. C. CURTIS July 3, 1951 ROCK DRILL.

3 Sheets-Sheet 1 Filed Feb. 23, 1946 15206721507: ciofin 6 arm alt J. C. CURTIS July 3, 1951 ROCK DRILL 3 Sheets-Sheet 3 Filed Feb. 23, 1946 Patented July 3, 1951 ROCK DRILL John 0. Curtis, Glaremont, N. H., assignor to- Joy- Manufacturing Company, a corporation of Pennsylvania Application February 23, 1946, Seri'aTNo. 649,496

19 Claims.

This invention relates to rock drills and more particularly to improved controlling means for the drilling motor for regulating the operating speedthereof automatically in accordance with load changes on the drill bit.

In rock drills, particularly those of the mounted type, it is desirable 'to' run the drilling motor-at a relatively slow idling speed when the drill bit is out of contact with the work, thereby to reduce wear and tear. In such drills, when the drill bit is being fed up to the work or being retracted from the work, it is desirable that the speed of the drilling motor be reduced thereby to avoid the excessive vibrations such as are set up by the drilling motor when the latter is running at relatively high speed without load. It is also desirable in such drills to run the drilling motor at a relatively slow speed during spotting of a hole, and when the drill bit is substantially resisted by the work, as during normal drilling, the drilling motor should be operated at full speed. In conventional rock drills of the mounted type, the speed of the drilling motor is usually manually controlled by a throttle valve which regulates the flow of pressure fluid to the drilling motor, and ofttimes the operator cannot foresee the varying conditions encountered by the drill bit and therefore does not properly regulate the motor speed, particularly during retraction of the drill bit-from a drilled hole so that considerable wear and tear occurs due to running of the drilling motor at full speed while the drill bit is substantially free-from the work.

It is an object of the present invention to provide an improved rock drill embodying improved ing speed of the drilling motor in accordance with the resistance of the drill bit by the work whereby the operating speed of the drilling motor will be automatically reduced when the drill bit is substantially out of contact with the work. Another object is to provide an improved automatic controlling means for the drilling motor of a rock drill whereby the operating speed of the drilling motor will be automatically increased when the drill bit is substantially resisted by the work. Yet another object is to provide improved means for automatically controlling the speed of operation of the drilling motor during spotting of a hole and for automatically increasing the speed of the drilling motor when the spotting operation is completed. A further object is to provide an improved control means for the throttle valve of the drilling motor of a rock drill for automatically regulating the motor means for automatically controlling the operat- 2L speed inaccordance with the resistance encountered by" the drill "bit. A still' further object isto provide improvedcontrol means responsi'veto changes in load on the drill bit of a rock drill for automatically regulating the flow of operating fluidtdthe drilling motor. Still anotherobject'is to provide an improved rock drill having improved automatic control'means for the drilling motor for reducing the operating speed thereof whenever the drill bit is underno substantial load and" for automatically increasing the-motor speedwhenever" the load'on the drill bit is sub: stantially increased. A'still iui'thenobject' is to provide improved control means for the drilling motor of a rock drill whereby the Jopcrating;"speed of the drilling motor is automatically controlled by the feeding pressure applied to "the drill bit by the drill feeding means. v is to provide an improved controlmeansfbr'the drilling, motor of a rock drillwhereb y the speed." of operation ofthe drilling. motor is automatically varied. as a result of variations the feeding) pressures imparted to the drillfbit by the reeding means. A still further objectis' to pro-vines improved automatic control. means for the drilling motor of arock drill, whereinrrelative trans.- latory movement betweenthe motor anditscontrol valve as a result. of load "changeson the motor, automatically effects:- regulation of. the These and now of pressure flu-id to the motor. other objects and advantages of the invention will, however; hereinafter more fully; appear.

In the accompanying. drawings there is. shown for. purposes of illustration-one-iforma whichthe J invention. may assume in practicer In: these drawings; i Fig. 1 'i's a side elevational view of' arock drill} in which an illustrative form ofi'the invention -i's embodied.

Fig. 2=isa view incentral longitudinal vertical section taken through the rock d'rill shown-in- Fig; l. r

Fig; 3 is a fragmentary view partially in side elevation and partially in section; the view being taken on the plane of" Fig. 2- and showing details of the improved controlfmeans "forthcdrilling" motor; with the controlvalve in forward drilling position.

' Fig.4 is a horizontal sectional view stantially on line 4- l'ofFig. 3';

Fig; 5is a'sectional'view similar to Fig. 4*, but

partly broken awaygshowing the controliparts drill idling position.

Fig. 6 is a sectional view similar toFig. 5' show- Yet another object? taken sub- 2,559,282 I fi I 3 ing the control parts in drill retracting position with the drill bit under load.

Fig. 7 is an enlarged cross-sectional view taken substantially on line 'l-l of Fig. 1 and looking toward the rear end of the drilling motor.

Fig. 8 is a detail cross-sectional view taken substantially on line 88 of Fig. 3.

Fig. 9 is an enlarged cross-sectional view taken substantially .onj' line'9-9 of Fig. 1 and showing details of the improved control means.

Fig. 10 is a fragmentary sectional view taken,

on the plane of Fig. 3 showing the automatic control valve in wide open position during retraction of the drill bit from thedrilled hole and with the bit under load.

Fig. 11 is a sectional view similar to Fig. 10,

pressure fluid actuated, hammer type for percussively actuating a drill steel 3 carrying a usual drill" bit 4. The annmg'mctor is mounted for feeding movement 'back and forth along the guideways of a' guide frame 5 in turn mounted on a trunnion support '6 which is held in position by a'usual clamp I on a column bar 8. Supported at the forward end of the guide frame is a'conventional drill steel 'ce'ntralizer 9 for guiding the drill bit during spotting of a drill hole and after the hole is spotted, the centralizer is released from the drill steel and moved into a position out of the path of the drilling motor, all in a well known manner. disclosed is shown in a horizontally disposed drilling position "on a column bar, it will be evident that the same may be disposed in any desired drilling "position and'mounted inany suitable manner.

The feeding means for moving the drilling motor back and forth along the guide frame 5 comprises a reversible feeding motorll herein of a conventional pressure fluid actuated type. and keyed to the motor power shaft I2 is a worm I3 meshing with a worm Wheel I4 in turn keyed toa shaft I5 suitably journaled within a housing I6 secured to the guide-frame 5. Fixed to the shaft. l5v is a feed chain: sprocket H. which engages vand drives afeed chain l8. The feed chain is guided by a seriesofrollers-l 9 on the .drill guide frame and is secured at to a sliding frame. or support 2| bywhich the drillmg motor is carried.

h feeding mo r. I Lhasa usua a ly op-.

erable reversing valve 22 and the housing I6 carries a manually operable throttle valve 23 for controlling the supply ,of pressure fluid from a supply hose connection 24 to a flexible hoseconnection 25 lea d ing to theautomatic control means I for the drilling motor. The throttle valve 23 also controls the'supply of pressure fluid from the v supply hose connection'lfl to the feeding motor II, under the .control of the reversing valve 22. The sliding frame 2l'has longitudinal guideways 26 slidingly receiving longitudinal guides 21 on the drill guide frame 5 and the sliding frame is movable longitudinally" back and forth alon the guiding frame. The sliding frame 2| includes front and rear cross frame'members 28 and 29 rigidly secured b longitudinal tie bolts 30 tothe While the rock drill body of the sliding frame and the feed chain [8 is fastened at 20 to the front cross frame member 28 in the manner shown in Fig. 3. The drilling motor has a longitudinal sliding connection with the sliding frame, and to provide such connection, the cylinder 3| of the drilling motor has lateral guides 32 slidingly received in longitudinal guide- Ways '33 on the slidin frame 21. The drilling motor cylinder guides 32 have limited longitudinal movement in the guideways 33 of the sliding frame 2|, and these motor cylinder guides have opposite end abutment surfaces 34 and 35 adapted selectively to abut against the cross frame members 28 and. 29 respectively. When the feeding motor I l is operated to drive the feed chain [8 to feed the drilling motor 2 forwardly along the guide frame 5 toward the work, the rear cross frame member 29 may engage the rear abutment surface 35. on the drilling motor cylinder, and when the feeding motor is operated to drive the feed chain to move the drilling motor rearwardly along the guide frame 5, the front cross frame member 28 may engage the front abutment surface 34 on the cylinder of thedrilling motor. Since the structure above described is fully disclosed in my patcut above referred to, further discussion of the structural details thereof is herein unnecessary.

Now referring to the improved automatic control means I for regulating the operatingspeed .of the drilling motor in accordance with load changes on the drill bit, it will be noted that ar ranged in longitudinal bores in the front cross frame member 28 are plungers 31 which are urged rearwardly toward the abutment surface 34 by ,coil springs 38, while surrounding a rod 39 secured to the rear endof the drill cylinder 3| is a relatively heavy coil spring 0 Which acts forwardly on the rear abutment surface 35. The

combined strength of the springs 38 is substantially the same as that of the spring 40. The springs acting on the plungers 3'! and the oppositely acting spring 40 tend to hold the cylin der 3| of the drilling motor centrally longitudi nally on the sliding frame 2i with the abutment surfaces 34 and 35 equally spaced from the cross members 28 and 29 as shown in Fig. 5; and when the drilling motor is in this central position on the sliding frame, the automatic control means I is in its position wherein the drilling motor runs at a relatively slow idling speed as hereinafter" explained. The rod 39 is herein a controlrod'. and extends longitudinally rearwardly from the motor cylinder and is pivotally connected atits;

rear end at 4| to a lever 42 secured to a stem 43 of a rotary throttle valve 4 3 which may be of any -conventional design. The valve 54 is arranged in a transverse bore in' a valve block 45 rigidly secured by a bracket 46 to the rear end of the slid- The hose connection 25 is connected by a supply pipe 51 to a passage 48 which 7 communicates with the bore of the valve block, as

ing frame 2 I.

shown in Fig. 9. A diametrically opposite passage 49 in the valve block is connected by a pipe 50 to a flexible ho'se connection 5| leading to the inlet The drilling: motor has usual automatic fluid distributing passage of the drilling motor 2.'

valve means and usual ports and passages for supplying fluid to the opposite ends of the cylinder to effect reciprocation of the motor piston in a well known manner. The body of the valve t4 is traversed by relatively inclined passages'52 and '1 53 opening through one side of the valve, and Q these ports communicate with a wide-mouthed, port 54 at the opposite side of the valve. when", the drilling motor is held by the oppositely acting springs 38, 4.0 in its central position on the sliding frame 2| as shown in Fig. 5, the throttle valve 44 is held by the control rod 39 and lever 42 in its central position shownin Eig. 11, wherein communication of the passages 52, 53 is partially cut off from the passage 49 so that the drilling motor may run at a relatively slow idling speed. When the throttle valve 44. is in either of its opposite positions, as shown in Figs. 3 and 10, the pressure fluid supply to the drilling motor inlet passage is full on so that the drilling motor may run at full speed as will later be explained. A bypass is provided so that the flow of pressure fluid to the drilling motor may be manually operated at the operators will and .this bypass provides a bypass pipe connection 55 leading from the passage :28 to a T 5.6 in pipe connection 50 sothat pressure fluid may bebypassed around the automatic control valve 44. In the bypass connection 55 is a conventional throttle valve 51 having a manual control handle 58.. When the valve 51 is in an open position, fluid may flow from the pipe 41 through passage 48, bypass connection 50 and hose connection 5| to the inlet passage of the drilling motor to enable running of the latter at full speed regardless of the load on the drill bit. During normal operation of the rock drill during drilling, the bypassvalve 51 is closed.

The general mode of operation of the improved rock drill is as follows: When the guide frame 5 is properly set up at the working face and the drilling motor 2 is in its retracted position on the guide frame with the drill bit 4 out of contact with the work, the operator may open the valve 23 to supply pressure fluid to the drilling and feeding motors 2 and H. At that time, the drilling motor 2 is held in its central position on the sliding frame 2| by the oppositely acting coil springs 38 and 413 with the automatic throttle valve M partially closed, as shown in Fig. 10, and with the drilling motor running at a relatively slow idling speed. The control valve 22 for the feeding motor I may then beadjusted to supply pressure fluid to the feeding motor to feed the drilling motor through the chain connection l8 relatively slowly along the guide frame toward the work. When the drill bit contacts the working face and during spotting of the hole, the feeding pressure on the drill bit is maintained relatively light under the control of the feed valve 22, so that the drilling motor is still maintained substantially centrally on the guiding frame 2! by the coil springs 38, ill, and while the hole is being spotted the drilling motor runs at a relatively slow speed which may be slightly higher than the idling speed. When the hole is spotted and the drill bit is substantially resisted by the work and under substantial load by an increased feeding pressure thereon under the control of the feed valve 22, the rear spring 4|! is compressed until the rear cross frame member 29 engages the rear abutment surface 35 of the drilling motor cylinder 3|, as shown in Figs. 2, 3 and 4; and as the sliding frame 2| slides forwardly relative to the drilling motor, the automatic control valve is turned through the rod and lever to the wide open position shown in Fig. 3, thereby to effect full speed running of the drilling motor to effect drilling of the hole. If during the drillin operation it is desired to change drill steels or if the hole has been drilled to the desired depth, the valve 22 of the feeding motor I! may be turned to effect reversal of the feeding motor, thereby to effect reverse feed of the drilling motor along the guide frame to retract the drill bit from the drilled hole. If there is a substantial load on thedrill bit during retraction from the hole, the sliding frame 2] will be moved rearwardly by the feeding means, compressing the front springs 38 until the front cross frame member 28 engages the front abutment surface 34 on the motor cylinder, and as the sliding frame 2| slides rearwardly relative to the drilling motor to the position shown in Fig. 6, the automatic control valve Ml is turned by the .rod and lever to its opposite wide open position shown in Fig. 10, thereby to effect full speed running of the motor. In the event during the retraction of the drill bit from the hole the load on the bit becomes relatively light or when the drill bit has been withdrawn completely from the hole and is out of contact with the work, the front springs 38 will move the drilling motor again to its central position on the frame 2| and the automatic throttle valve 44 is turned through the rod and lever again to the partially closed position shown in Fig. 11 to effect running of the drilling motor at a relatively slow idling speed. It is accordingly evident that when the drill bit is out of contact with the work or if the load on the drill bit is relatively light, as during spotting of a hole, the drilling motor runs at a relatively slow speed and whenever the load on the drill bit becomes relatively great, as during normal drilling, or when the drill bit tends to stick in the hole during withdrawal, the drilling motor runs at full speed. By regulating the feeding pressure on the drill bit during drilling under the control of the valve 22 of the feeding motor, the speed of the drilling motor may be varied as desired. For example, the speed of the drilling motor may be regulated under the control of the feeding means to run at a speed somewhat higher than idling speed but substantially less than full speed during spotting of a hole. If it is desired to run the drilling motor at full speed regardless of the load on the drill bit, the operator may open the valve 5! of the bypass means so that pressure fluid may be bypassed around the automatic contrcl valve.

As a result of this invention, an improved rock drill is provided wherein the operating speed of the drilling motor may be automatically regulated in accordance with load changes on the drill bit. It will further be evident that the running of the drilling motor at full speed only when the drill bit is substantially resisted by the work, wear and tear due to high speed running of the drilling motor at no load is substantially reduced. Further, by regulating the operating speed of the drilling motor by varying the feeding pressure imparted to the drill bit, spotting of a hole may be efficiently effected. By the provision of the manually controlled bypass means, it is possible to control the drilling motor as desired wholly independently of the automatic control means. Other advantages and modes of use will be clearly apparent to those skilled in the art.

While there is in this application specifically described one form which the invention may assume in practice, it will be understood that this form of the same is shown for purposes of illustration and that the invention may be modified and embodied in various other forms without departing from its spirit or the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent is:

1. In a rock drill, the combination comprising adrilling motor for actuating a drill bit, reversible feeding means for feeding said motor toward and from the work, means for operatively connecting said feeding means to said motor and yieldable irrespective of the direction of feed, means for limiting the degree of yielding movement in either direction, and means effective during either direction of feed and actuated upon such relative yielding movement for regulating the motor speed.

2. In a rock drill, the combination comprising a fluid actuated drilling motor for actuating a drill bit to effect drilling of a hole, feeding means for said motor for feeding the drill bit relative to the work, means for operatively connecting said feeding means to said drilling motor and embodying a connection permitting limited relative longitudinal movement between said motor and an element of said connecting means, manually operable means for controlling said feeding means to effect application of varying feeding pressures to the drill bit, control valve means carried by said element of said connecting means and actuatable to regulate the flow of operating fluid to said motor to vary the operating speed of the latter, and operating means operatively connected to said control valve means and to said motor and responsive to such relative longitudinal movement resulting from such varying feeding pressures for variably actuating said motor control means.

3. In a rock drill, the combination comprising a drilling motor element for actuating a drill bit to effect drilling of a hole, feeding means for said motor element for feeding the drill bit relative to the work, means for operatively connecting said feeding means to said drilling motor element and providing for limited relative movement between the terminal element of said feeding means and said motor element, control means carried by oneof said elements and actuatable to regulate the operating speed of said motor element, and operating means operatively connected to said control means and to said other element and actuated upon such relative movement between said terminal element and said motor element in accordance with load changes on the drill bit during the drilling operation for variably actuating said motor element control means.

4. In a rock drill, the combination comprising a guide providing longitudinal guiding means, a support movable longitudinally along said guiding means, means for moving said support along said guiding means, a drilling motor for actuating a drill bit to effect drilling of a hole, means for mounting said motor on said support for limited translatory movement inalongitudinal direction relative thereto, control means carried by said support and actuata-ble to regulate the operating speed of said motor, and means operatively connected between said drilling motor and said control means and actuated upon such relative translatory movement between said support and said motor for actuating said motor control means.

5. In a rock drill, the combination comprising a guide providing longitudinal guiding means, a support slidable longitudinally along said guiding means, means for sliding said support along said guiding means, a drilling motor slidably guided on said support for limited movement relative thereto longitudinally of said guiding means, yieldable means, arranged between said motor and said support for resisting such relative longitudinal movement between said drilling motor and said support, control means carried by said support and actuatable to regulate the 011-: crating speed of said drilling motor, and means operatively connected to said control means and to said motor and actuated upon such relative longitudinal movement between said motor and said support for actuating said motor control means.

6. In a rock drill, the combination comprising a guide providing longitudinal guiding means, a support slidable longitudinally along said guiding means, drilling motor slidably guided on said support for limited movement relative thereto longitudinally of said guiding means, yieldable means arranged between said motor and said support for resisting such relative longitudinal movement between said drilling motor and aid support, feeding means operatively connected to said support for feeding said drilling motor toward the work through said yieldable means, control means carried by said support and actuatable to regulate the operating speed of said motor, and means operatively connected to said control means and to said motor and actuated upon such relative longitudinal movement between said motor and said support for actuating the operating speed of said motor, means 013- eratively connected to said motor control means and to said motor and actuated upon such relative longitudinal movement between said motor and said support for actuating said motor control means, and means for manually controlling said feeding means to regulate the feeding pressures imparted to said support for controlling the amount of such relative longitudinal movement between said motor and said support whereby said motor control -means is variably actuated.

8. In a rock drill, the combination comprising a drilling motor for actuating a drill bit to effect drilling of a hole, feeding means for said drilling motor for feeding the drill bit relative to the work, means for controlling said feeding means to regulate the feed pressures applied to the drill bit, control means actuatable to regulate the operating speed of said motor, and means operatively connected to said motor control means and to said motor and responsive to variable feeding pressures applied to the drill bit for variably actuatingsaid motor control means.

gitudinal sliding connection between said feeding means and said motor, means connected between said feeding means and said motor for limiting the sliding movement afforded by such connection, control means carried by an element of saidsliding connection and relative to which said motor has longitudinal sliding movement, said control means actuatable to regulate the operating speed of said motor, and means operatively connected to said motor control means and to said motor and actuated as the result of such sliding movement for actuating said control means.

10. Ina rock drill, the combination comprising a guide providing longitudinal-guiding means, a drilling motor movable longitudinally along said guiding means, means for feeding said motor along said guiding means, means providing a l'o'ngitudinal sliding connection between said feeding means and said motor, said sliding connection including a sliding frame guided for movement longitudinally along said guiding means and hav- 'ing a longitudinal guideway, said motor having a guide slidably received in. said frame guideway, means arranged between said motor and said frame for limiting relative sliding movement between said motor and said frame, control means carried by said sliding frame and actuatable to regulate the operating speed of said motor, and means operatively connected between said motor and said motor control means and actuated as the result of such relative sliding movement between said sliding frame and said motor for actuating said control means.

11. In a rock drill, the combination comprising a guide providing longitudinal guiding means, a drilling motor element movable longitudinally along said guiding means, means for feeding said motor element along said guiding means, means providing a longitudinal sliding connection between said motor element and said feeding means including a sliding frame guided for movement along said guiding means and to which said feeding means. is operatively connected, said frame having a longitudinal guideway, and said drilling motor element having a guide slidably received in said frame guideway, control means carried by one of said elements and actuatable to regulate the operating speed of said motor element, and means operatively connected between said drilling motor element and said motor element control means and actuated as the result of such relative sliding movement between said elements for actuating said control means.

12. In a rock drill, the combination comprising a drilling motor for actuating a drill bit to effect drilling of a hole, means operatively connected to said motor for feeding the drill bit toward the work, control means actuatable to regulate the operating speed of said motor, and means operatively connected to said motor control means and to said motor and responsive to varying load changes on the drill bit for variably actuating said control means.

13. In a rock drill, the combination comprising a drilling motor for actuating a drill bit to effect drilling of a hole, means operatively connected to said motor for feeding the drill bit toward the work, control means actuatable to regulate the operating speed of said motor, means operatively connected to said motor control means and to said motor and responsive to load changes on the drill bit for variably actuating said control means, and means operable at will for regulating the operating speed of said motor independently of said variable control means and the load changes on the drill bit.

1%. In a rock drill, the combination comprising a guide providing longitudinal guiding means, a drilling motor movable along said guiding means, feeding means operatively connected to said motor for feeding the latter along said guiding means, means providing a longitudinal sliding connection between said motor and said feeding means, yieldable means arranged between said motor and said feeding means for resisting such relative sliding movement between said motor and said feeding means, means connected between said feeding means and said motor for limiting such relative sliding movement, control means carried by an element of said sliding connection and actuatable to regulate the operating speed of said motor, and means operatively connected to said motor control means and to said motor and actuated upon such relatively sliding movement for actuating said control means.

' 15. In a rock drill, thecombination comprising a drilling motor for actuating a drill bit to eifect drilling of a hole, feeding means for said motor feeding the drill bit relative to the work, means for operatively connect ng said feeding means to said motor and providing a yieldable connection between said feeding means and said motor, means connected between said feeding means and said motor for limiting the amount of relative yielding movement therebetween, control means carried by an element of said yieldable connection and actuatable to regulate the operating speed of said motor, and means operatively connected to said motor control means andto said motor and actuated upon such relative yielding movement between said motor and said feeding means for actuating said control means.

16. In a rock drill, the combination comprising a drilling motor, control means actuatable to regulate the operating speed of said motor, and means operatively connected to said motor control means and responsive to load changes on said motor for variably actuating said control means during the drilling operation, and embodying means connected to said control means and to said motor for positioning said control means to reduce the operating speed of said motor to effect idling thereof whenever said motor is running without load.

1'7. In a rock drill, the combination comprising a drilling motor element for actuating a drill bit to effect drilling of a hole, reversible feeding means for said motor element for feeding the drill bit toward and from the work, means for operatively connecting said feeding means to said motor element and including a feeding element having limited movement with respect to said motor element irrespective of the direction of feed, means arranged between said feeding means and said motor element for limiting the amount of such relative movement in either direction, control means carried by one of said elements and actuatable to regulate the operating speed of said motor element, and means operatively connected to said motor control means and to said other element and actuated upon such relative movement during either direction of feed for actuating said control means.

18. In a rock drill, the combination comprising a drilling motor, control means actuatable to vary the operating speed of said motor, and means operatively connected to said motor control means and to said motor and responsive to load changes on said motor for variably actuating said control means, said control means embodying means for automatically reducing the operating speed of said motor to effect idling thereof whenever said motor is running without load and for automatically increasing the operating speed of said motor as the load on said motor increases.

19. In a rock drill, the combination comprising a pressure fluid actuated, hammer type drill motor for percussively actuating a drill bit to effect drilling of a hole, feeding means, for said motor for feeding the drill bit relative to the work and embodying a connection between said feeding means and said motor permitting limited relative longitudinal movement therebetween, control means for said feeding means to apply variable feeding pressures to the drill bit, means carried by an element of said connection and including a control valve for said motor and actuatable to vary the flow of operating fluid to said motor to regulate the operating speed thereof, and operating connections between said motor and said motor control valve for variably actuating the latter in accordance with varyingfeeding pres- 12 sures on the drill bit whereby when the feeding pressure is increased the operating speed of the motor is increased and vice versa.

JOHN C. CURTIS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,760,316 Osgood May 27, 1930 2,221,118 Smith, Jr., et al. Nov. 12, 1940 2,236,700 Slater Apr. 1, 1941 2,322,237 Johansen June 22, 1943 2,338,625 Curtis Jan. 4, 1944 2,375,991 Hoflar May 15, 1945 

